Pervaporation is a commercially-practiced membrane separation process in which a non-porous membrane is contacted with a multicomponent liquid feed, resulting in the selective absorption of one or more of the species from the feed. These sorbed species permeate across the membrane under the influence of a concentration gradient that is produced by evaporating the sorbed molecules from the product side of the membrane using a vacuum or sweep gas. Permeate vapor is then condensed and recovered as a liquid. Vapor permeation differs from pervaporation in that the feed is already in the vapor phase.
The concept of pervaporation has been recognized for a long time (Mitchell, J. V.; J. Roy. Inst., 2 (101), 1831, 307); however, only in the past 40 years has development of this technology as a process taken place. Binning and coworkers at American Oil Co. (Binning, R. C.; R. J. Lee; J. F. Jennings and E. C. Martin; Ind. and Eng. Chem., 53(1), 1961, 45-50; Binning, R. C et al., U.S. Pat. No. 2,970,106, January 1961) were the first to suggest, based on their experiments with organic liquid mixtures, that pervaporation had commercial potential. During the next two decades, however, most of the work on pervaporation focused on alcohol (ethanol)-water separation, since this demonstrated that pervaporation could achieve something not possible by ordinary distillation-breaking of the ethanol-water azeotrope. SETEC and GFT (now part of Le Carbone-Lorraine) in Germany commercialized pervaporation membrane systems based on composite polyvinyl alcohol membranes for this application in the 1980's as reported by Bruschke and coworkers (Bruschke, H. E. A.; G. F. Tusel and R. Rautenbach; ACS Symposium Series, 281, 1985, 467-478); and there are numerous commercial facilities around the world with capacities as large as 150,000 liters/day using this technology. The current invention relates to an improvement in both pervaporation systems and vapor permeation systems and specifically an improved method of maintaining a low partial pressure of permeate on the permeate side of the membrane. U.S. Pat. NO. 5,753,008, which is incorporated herein by reference, which discloses a vapor permeation process, teaches that there are three known ways used for maintaining a sufficiently low partial pressure of permeate on the downstream side of a membrane: (a) the vacuum method, (b) the dilution method and (c) the countercurrent sweep method. Most commercial pervaporation systems use the vacuum method. Small systems use vacuum pumps while larger systems, of necessity, use multi-stage steam ejectors; and the cost of operation with the latter becomes a significant part of the total package cost.
There are problems with both methods of maintaining vacuums. Vacuum pumps achieve excellent vacuums (&lt;10 torr), but are expensive to maintain. Unfortunately, the better the vacuum, the harder it is to condense the permeate; and many commercial pervaporation systems have expensive refrigeration systems as part of their condensers. Furthermore, because it is impossible even at very low temperatures to totally condense the permeate (especially at &lt;10 torr), vacuum pump oil may become contaminated with permeate, thus requiring more maintenance. Vacuum pumps cannot be used for large systems, because their displacements are generally too low.
Steam ejectors are simpler, but are limited in the level of vacuum they can achieve at reasonable steam flow rates. Three-stage ejectors are normally required for pervaporation systems, thus adding to their complexity. Furthermore, a small amount of permeate is mixed with the steam condensate; and, while for most organic systems, the water phase and the organic phase separate, there is always a finite concentration of permeate dissolved in the water condensate that still needs to be removed prior to discharge into natural waterways.
It would be desirable to have an inexpensive process for the selective pervaporative separation of either aqueous or organic feeds that is applicable over a wide range of feed rates. Such a process would represent a significant advance in the art.
It is therefore an object of the present invention to provide a highly efficient method and apparatus for pervaporation or vapor permeation that can be used in both small and large process applications.
It is a further object of the present invention to provide a highly efficient method and apparatus for treatment of either aqueous or organic feeds. It is still a further object of the present invention to provide a highly efficient method and apparatus that provides superior performance using different module designs (hollow fiber, spiral wound or stacked flat).
These and other objects, which will become apparent to one of ordinary skill, are summarized and described in detail below.